Capacitance material level probes of the type to which the present invention is directed typically comprise a central rod surrounded by a first layer of insulation, with the rod tip being exposed to act as a measurement antenna when mounted within a storage vessel. A tubular guard shield surrounds the first insulation layer and is itself surrounded by a second layer of insulation. A portion of the tubular guard shield adjacent to the rod tip is exposed and is adapted to be electrically energized separately from the rod so as to direct probe energy outwardly into the tank rather than through any material which may be coated onto the probe body. The second insulation layer is clamped within a nipple which is adapted to be threaded into a gland on a material vessel wall with the probe rod and guard shield projecting into the tank as described.
Measurement probes of the described character heretofore proposed in the art typically include the insulation layers and the guard shield as separate tubular elements telescopically received over the central rod. To obtain desired sealing within a pressurized vessel, it has often been necessary to crimp or squeeze the tip of the tubular guard shield around the first insulation layer to close off a leakage path therebetween. The separate tubular insulation elements and assembly thereof to the probe rod are expensive, and it is now proposed to perform the assembly operation and provide the necessary insulation in a single step by injection molding insulation material surrounding and between fixtured guard shield and probe elements. However, considerable shrinkage is encountered with thermosetting plastics suitable for this application, and crimping of the guard shield tip, which is undesirable in any event because it requires an extra assembly operation, is not entirely effective.
An object of the present invention, therefore is to provide an improved capacitance-type probe of the character described which is adequately and securely sealed at the tip of the guard shield.